This invention relates to an aluminum nitride body and the method of producing such a body and, more particularly, relates to an aluminum nitride body having features and vias which have a graded metallurgy structure and to a method for producing such an aluminum nitride body.
Aluminum nitride has been of interest recently for electronic packaging applications because of its high thermal conductivity, thermal expansion matching with silicon, low dielectric constant (8.5) and high electrical resistivity.
The present invention is particularly suitable for co-fired electronic packages, also known as substrates. In one co-firing process, the aluminum nitride is formed into greensheets (comprised of aluminum nitride particles in an organic binder), vias are punched, metallization paste (comprised of metallic particles in an organic binder) is screened or extruded onto the greensheets and into the vias, the greensheets are stacked and laminated to form a substrate in the green state, and then the green substrate is sintered to densify the aluminum nitride layers and the metallization. "Co-fired" means that the metallic paste is sintered during the same sintering schedule as the aluminum nitride body. The metallization for aluminum nitride substrates is typically tungsten but may also be molybdenum or a mixture of tungsten and molybdenum. In addition, instead of forming the aluminum nitride body by using greensheets, dry pressing may be used to form the aluminum nitride body.
The co-firing of aluminum nitride substrates with tungsten metallurgy requires matching of the sintering behavior of aluminum nitride and tungsten. This can be achieved by introducing various sintering additives to the aluminum nitride and tungsten powders. Generally, the tungsten powder shows sintering onset at a lower temperature compared to aluminum nitride and hence the sintering additives added to tungsten retard the onset of tungsten sintering. The sintering additives may also reduce the final sintered density of tungsten, possibly resulting in porous tungsten. This type of metallurgy, though useful for internal metallization, cannot be used to produce input/output (I/O) pads or surface metallization due to its porous nature. Use of porous metallization results in a non-hermetic substrate which is undesirable because any wet processing subsequent to the formation of the I/O pads will allow liquids to penetrate within the substrate which can degrade the manufacturability and performance of the substrate. Use of pure tungsten powder that will sinter to high density (and therefore be hermetic) for external metallization results in early sintering of tungsten, leading to debonding at the aluminum nitride/tungsten interface before the aluminum nitride sinters to full density, giving very low adhesion strength.
The solution to this dilemma involves addressing two problems that are interdependent, namely, shrinkage matching of tungsten metallization to that of aluminum nitride during co-firing by choosing the right sintering additives and ensuring that fully dense tungsten results notwithstanding the presence of sintering additives that retard the sintering of tungsten.
Thus far, this solution remains unfulfilled.
Okuno et al. U.S. Pat. Nos. 4,695,517 and 4,800,137, the disclosures of which are incorporated by reference herein, have proposed a composite layer on an aluminum nitride body in an effort to increase the bond strength between the aluminum nitride body and the metallization. Okuno et al. propose overlapped layers on the aluminum nitride body of tungsten (or molybdenum) plus aluminum nitride followed by a layer of tungsten (or molybdenum). Okano et al. do not appear to appreciate that the first layer may be porous, thereby necessitating that it be compensated for lest a non-hermetic substrate should result. Even if the first layer of tungsten (or molybdenum) plus aluminum nitride is nonporous, it must be completely covered by the overlying layer of tungsten (or molybdenum) because any subsequent plating will not adhere well to the first layer, resulting in damage to the entire plated layer. A plated layer is usually disposed over the tungsten (or molybdenum) layer in order to facilitate I/O pin or wire bonding.
Sato et al. European Patent Application 0 574 956, the disclosure of which is incorporated by reference herein, propose an intermediate co-fired layer of tungsten or molybdenum plus titanium nitride followed by metallization such as by nickel plating.
Harada et al. U.S. Pat. Nos. 4,980,239 and 5,096,749, the disclosures of which are incorporated by reference herein, discloses a composite layer structure on an aluminum nitride substrate comprising sequential thin film layers of titanium, tungsten or molybdenum, and nickel. Thereafter, the composite structure is heated to a temperature high enough to cause a reaction between the titanium layer and the aluminum nitride body, resulting in an intermediate layer of aluminum titanium nitride which improves the adhesion of the metallization.
Iio et al. U.S. Pat. Nos. 4,840,853 and 4,892,703, the disclosures of which are incorporated by reference herein, propose an aluminum nitride body having an intermediate layer comprising aluminum, nitrogen and oxygen, and a metallized layer of tungsten or molybdenum. The intermediate layer is necessary to increase the bonding strength of the metallized layer. It is not clear from the description how this structure is formed.
The internal vias of the substrate present a different problem. Pure tungsten or molybdenum will not adhere well to the walls of the vias. So-called "rattling vias"may be obtained where the metallic via floats within the via opening which may result in cracking around vias. Accordingly, the present inventors have recognized that a graded metallurgy may be useful for the internal vias.
Knickerbocker et al. U.S. Pat. 5,260,519, the disclosure of which is incorporated by reference herein, discloses a multiple layer/via structure wherein the composition of the vias indifferent layers is graded from metallic to a mixture of ceramic and metal. There is no teaching of grading the via composition in a single via opening.
Dolhert, et al., U.S. Pat. 5,200,249, the disclosure of which is incorporated by reference herein, discloses an hermetic via composition for an aluminum nitride substrate consisting of a mixture of aluminum, nitride and tungsten or molybdenum.
Panicker, et al., U.S. Pat. 4,942,076, the disclosure of which is incorporated by reference herein, discloses a composite via composition for an alumina substrate. The substrate is sintered with tungsten paste which forms a porous via. Copper is later infiltrated into the porous tungsten.
In view of the above attempts at composite and graded structures, it is a purpose of the present invention to have a metallurgical structure with good adhesion strength to accommodate the bonding of I/O pins and wire bonds.
It is another purpose of the present invention to have a metallurgical structure that is hermetic with respect to the ambient environment.
It is yet another purpose of the present invention to have a metallurgical structure that is particularly useful for making composite vias.
These and other purposes of the invention will become more apparent after referring to the following description in conjunction with the accompanying drawings.